This invention relates to processing of digital bits in an adaptive system which employs the generation of pseudoerrors. More particularly, the present invention relates to a circuit for ensuring that the number of pseudoerrors generated is controllable to lie within a range satisfactory for the operation of the adaptive system.
The circuit of the present invention finds particular utility in an adaptive equalizer of the type described by the present inventor in U.S. patent application Ser. No. 517,317, filed July 26, 1983, now U.S. Pat. No. 4,538,283 issued Aug. 27, 1985, and entitled "Adaptive Equalizer Suitable for Use with Fiber Optics". In that application, the present inventor describes how a digital pulse received in a transmission system and transformed to baseband can be adaptively equalized for transmission distortion by a control loop which includes processing of the received digital pulse by an overcompensated equalizer and an undercompensated equalizer. The overcompensated and undercompensated pulses are each input to an associated pseudoerror comparator and detector which generates a pseudoerror if the amplitude of the pulse lies within a selected pseudoerror region. For NRZ pulses, this region extends above and below the bit slicing level for one/zero determination. Thus, pseudoerrors are produced by pulses which are not clearly a logic one or a zero.
The term "pseudoerror" is used to emphasize that the errors are artificially produced, and that their presence does not mean that there are errors occurring in the main bit identification circuitry controlled by the adaptive loop. In the system which is the subject of the referenced patent application, the errors are artificial in two respects. First, they are generated by over- or undercompensated equalizers. Second, the designation of certain bit pulse amplitudes as constituting errors is done strictly so as to satisfy the requirements of the adaptive control loop.
The definition of that range of bit pulse amplitudes which constitute errors is the problem to which the current invention is directed. The adaptive equalization system of the referenced patent application requires that the rate of pseudoerror generation lies within certain roughly defined boundaries. If very few pseudoerrors are generated, then the adaptive mechanism will tend to flip from one extreme to the other, depending upon whether it is the overcompensated or undercompensated part of the system which is generating the pseudoerror. If too many pseudoerrors are generated, it will be difficult for the system to detect a difference between the over and undercompensated sources of pseudoerrors. It is expected that other bit pulse processing systems which can employ pseudoerror generation will benefit from having the rate of error generation be within certain boundaries.